Resonance and standing-wave impedance measuring line



July 25, 1950 2,516,169

RESONANCE AND STANDING-WAVE IMPEDANCE MEASURING L INE M. s. wQNG :ssheets-sheet 1 Filed Dec TOR.

' A fron/vtr wf/vr- M. s. woNG- July 2-5, 1950 4 RESONANCE ANDSTANDING-WAVE IMPEDANCE MEASURING LINE Filed Dec. 10, 1947 BY WW M. S.WONG July 25, 195o REsoNANcE AND STANDING-WAVE IMPEDANCE MEASURING LINE:s sheetsheet s Filed Dec.

IN VEN TOR. M//V /V' UW- Patented July 25, 1950 RESONANCE ANDSTANDING-WAVE IMPEDANCE MEASURING LINE y Ming S. Wong, Dayton,'0hioApplication December 10, 1947, Serial No. 790,751

n 2 Claims. v (Cl. 175-183) (Granted under the act of March 3, 1883, as

amended April 30, 1928; 370 0. G. 757) The invention described hereinmaybe manufactured and used by or for the Government for governmentalpurposes without payment to me of any royalty thereon.

This invention relates to the measurement of impedances at highfrequencies of the order .of 3.00 to 5000-megacycles .per rsecond andparticularly to those methods of impedance measurement that involveconnecting the .impedance to be measured to a transmission line andmaking measurements on the viline which may be used to calculate thevalue-of. .the impedance.

It is the object of the invention to provide an apparatus for measuringa wide range of pedanc'es witha high `degree of accuracy.

More specifically itis the object of the invention -to provide a highyfrequency transmission line that is particularly suited to themeasurement of impedances by the resonant-current method, but that alsomay be used .to .measure im-pedances by the standing-wave method.

The impedance measuring .line to be described is particularly adapted tothe measurement of antenna impedances lout may be used equally well tomeasure the impedance of any element or network at high frequencies.

Spec-inc embodiments of the invention are shown in the drawingsin'which:

Fig. 1 isa plan view ofthe complete impedance measuringline,

Fig. 2 shows a cross-sectional elevation of that part of the linedesignated .by bracket A in Fig. l

Fig. 6 is a cross-sectional elevation of that part of the lineincludedinbracket C of Fig. 1 at anenlarged scale, l

Figs. 7, 8v and 9 show modifications tov permit the use of atransmissionline of' the'two conductor shielded type.

Fig'. 10 is an equivalent circuit ofthe impedance measuringv line when.operated in accordance with the resonant-current method.

Fig. 11 illustrates the measurementsto be made on the line in theresonant-current method.

Fig. 12 shows the equivalent circuit of .thei-mpedance measuring linewhen :operated inaccordance with the standing-wave method. v

Fig. 13 illustrates the measurements'tolbe made on the line in using thestanding-wave method of impedance measurement.

Fig. 14 shows Va suitable Voltage indicating device Vfor use with theimpedance measuring line when operated in accordance with the standing-Wavermethod.

In the Aplan view of `the complete impedance measuring line shown inFig.l, I represents a rigid base of cast metalhaving a flat machined uppersurface iupon which the various elements of the line are mounted. Thetransmission line 2 consistsof anouter conductor inthe form of atu'bemade Vof a highly conductive material. such' as brass and an inner`conductor mounted con.

centrically with the outer conductor. The extension of the centerconductor may be seen to the right at y3. That part of Vthe innerconductor located within the.outerconductor is also made of a highlyconductive material such as brass and, if desired, 'the .inner surfaceof the outer conductor and the outer surface of the inner conductor maybe plated with silver in order to further increase losses. v

About one-half of the outer conductor of transmission line 2 ispressedlinto a metallic block 4 which is xedly mounted on base I and serves tosupport one lend of vtransmission line 2. That part of the transmissionlineinside block 4 is slotted, `as is also the upper surface of block 4'as shown at 5, to permit a. probe to be inserted intov the spacebetweentheouter and inner conductors blocks-d and are shown in Figs. 2and 3.

Referring to Fig. l2 the block 6 is provided with a tongue lll which ntsthe slot 5 and extends downward so that its lower'surface is nus-h withthe inner surface of the outer conductor of the transmission line, thuspreserving the inner conthe conductivity and reduce the block to theplug fitting I (Fig. 3).

anale@ tour of the line under block 6.- In order to preserve the innercontour of the line throughout the length of block 4 various lengths ofmetallic covering strips II, having 2 cross sections as shown in Fig. 3,are provided for lling the slot 5 on either side of block 6 once theposition of block 6 for a particular measurement has been determined.Probe I2 having reduced lower portion and a threaded upper portion ispositioned in hole I3 by threading the upper Vportion into the conductorI4 which extends horizontally across With this arrangement the probe maybe adjusted up or down to regulate the degree of coupling to thetransmission line. Cover screw I6 is provided to permit adjustment ofthe probe and to maintain complete shielding. The center conductor 3 issupported at the end 9 of the transmission line .by insulator I'I whichmay be made of any suitable material having low loss at high frequenciessuch as polystyrene. Additional discs of lowloss inf sulating materialsuch as I8 and I3 may be provided for supporting the center conductor ifnecessary. These discs Lare slotted at the top to permit passage ofprobe I2. The other end of transmission line 2 is rigidly held bysupport 3I which is xed to base I.

Referring again to Fig. 1 a metallic plunger 20 is provided that fitsinto the end of transmission line 2 and serves to short circuit thetransmission line at the inner face 2l of the plunger. The plunger has ahole lengthwise thereof at its center to permit passage of the extendedinner conductor 3 of the transmission line. Movement of plunger 2Userves to change the electrical length of the transmission line 2. Inorder to arrange for moving the plunger 20, and also for movingtherewith all associated electrical apparatus which should have a xedposition relative to the plunger, the travelling base 22 is provided.This base is a rigid metallic casting and is arranged to slide on theupper surface of base I, being guided and held in place thereon by gibs23. An extension 24, which is also a metallic casting, is xed to theside of travelling base 22 and lmoves therewith. The outer end ofplunger 20 is tightly pressed into a hole in the extension casting 24which supports this end of the plunger and serves to move the plungerinto or out of transmission line 2 as the base 22 is moved. Theextension 24 carries a hand wheel 25 which may be pushed in to engageshaft 26 on which is mounted a pinion (not shown) that engages rack 2lfor rapid movement of the plunger 20 and travelling assembly 22-24'. Inorder to provide for a fine adjustment of the position of plunger 20 alead screw 28 driven by hand wheel 29 is provided. To` use the neadjustment the arm 30 mounted on base 22 is rotated to cause a camactuated half nut (not shown) to engage the lead screw 28. Rotation oithe lead screw then acts to move the travelling base 22 and, as aresult, the plunger 20. The scale 'I5 serves to indicate the length ofline between end 9 and the face 2I of plunger 20.

The details of plunger 20 and the plunger end of transmission line 2 areshown in Fig. 4. The plunger 20, except near the short-circuiting end,has an outside diameter slightly less than the inside diameter of theouter conductor of transmission line 2, and the diameter of the holethrough. the center of the plunger is made slightly greater than thediameter of the center conductor 3, in order to allow free movement ofthe plunger within the transmission line. However,

4 for some distance back from the short-circuiting` end 2I of theplunger a heavy plating of a hard metal such as hard chromium is appliedto the outer surface of the plunger and the inner surface of the holethrough the center of the plunger. The plated end of the plunger is thencarefully machined and polished to have a very accurate and close iitwith the inner surface of the outer conductor and the outer surface ofthe center conductor of the transmission line. This arrangement providesa veryeffective shore-circuit between the conductorsof Vthe transmissionline and due to the fact that contact is made between the soft metal ofthe line conductors and the hard metal on the plunger the wear is veryslight and the effectiveness of the short circuit is maintainedthroughout a long period of use. This ltype Yof short circuiting meansis found to maintain its uniformity over a longer period of time thanthe usual spring nger type and also the point on the line at which shortcircuiting occurs is more sharply dened and coincides very nearly withthe plane face 2l of the plunger. A felt seal 32 is provided at the openend of the transmission line to prevent dirt being carried into the lineby the plunger.

A small coupling loop 33 is positioned on the Surface 2l of the plungertoprovide means for taking energy from or supplying energy to thetransmission line depending upon the way in which the line is used tomeasure impedances as will be explained later. Connected with this loopis a concentric transmission line 3G having an inner conductor 34 and anouter conductor formed by the inner surface of a longitudinalcylindrical hole in the metallic-plunger. The center conductor 34 ispositioned in the hole by suitable insulating material 35 having lowloss at high frequencies.

The yplunger 20 may be fabricated by rst machining semicircular channels31 and 38 .and a receptacle 39 for :one end of loop 33 in two pieces ofmetal as shown in Fig, 5. The two pieces of metal may then be welded orsweated together and turned to the proper size.

Fig. 6 showsV a cross section of extension 24 taken along a verticalplane through the center of plunger 2U.- Transmission line 38 afterleavy, ing the plunger 2li, passes upward through .cast- 50' ing 24 intohousing 40. This part of transmission line 35 is provided with an outermetal shield 4I, which serves as the outer conductor, a functionperformed by the body of the plunger forv that part of the line withinthe plunger. The shield 4I is electrically connected to the plunger at42 and its inside diameter should equal the diameter of the transmissionline passageway in the plunger so that the line is uniform throughoutits length. The shaft 26 in casting 24 carries pinion 43 which engagesrack 21 (Fig. l) for rapid movement of the plunger as previouslyexplained.

Referring again to Fig. l the transmission line 36 passes from housirrg4`Il through' ileXible outer shield 45 into housing 44 which is rigidlymounted on travelling base 22. Elements 46`and 41 are concentric shortcircuited stub lines having Slots 48 and 49 through which a sharppointed instrument may be inserted to move an adjustable short Ycircuitingmeans (not shown) yfor varying the lengths of the lines. Theslots 48 and 49 may be closed after adjustment if desired by the` use ofcovers of the type shown at II in Fig. l3. Stub line 46 is rigidlymounted on travelling base.22

by supports 50 andal and has; -its inputend.

' and 5.4.-y and has its input end located inside housing `55r which .is.also fixed to plate 52. .Hand .wheel `5.5 `is mounted on plate 52 anddrivesa pinion'which engageswarackgon travelling v,basev22 below .plate52 (not` shown) so that by rotating wheel 56 plate .5.2, which carriesline 4l andfhous- Iing. 55,.may be moved relative to travelling ibase22. Gibs .El serve` to guide and hold plate=52fin place .on base 22. n

An adjustable lengthzof .concentric transmissionfline 58 is situated.between .housing 44 and housing 5.5. This line consists of alargersection 58 rigidly-mountedfonibase 2? bysuppOrt-Bl and housing eilandslidable over smaller section 6.9 which is :fixed to. hcmsing 5.5 .andltherefore moves with'plate 52. Both the outer and inner conductorsofsection 5.9 t over vthe louter and inner conductors respectively ofsection. 8 8 .so that two ytelescoping sections are formed'. By rotatingvwheel Edsection 58 moves into or out of section 59 thus varying thetotal length of composite line 53.v Variable length lines of-this typeare well ,known inv the art and `are sometimes referred to as linestretchers."

The housing 55v .contains .a suitable detector for high lfrequenciessuchas a .fcrystal detector. Inside housing 55 the detector, the Ainput endof transmission line el and yone end .of adjustable transmission line 58are connected in parallel. Inside housing li the other end of adjustableline 5.8,. the input end of line d8 and the end of transmission line 35are connected in parallel. The stub lines 36 and il and adjustable line58 form an impedance vmatching .network and by proper adjustment ofthese elements the impedance appearing at the end of transmission line36 mayl be taken from plug fitting 82 and applied'to any.

suitable type of direct -current or audio fre quency indicating device.

An important feature ofthe impedance meas-l uring line is that norelative motion of any highfrequency circuits located between loop '33(Fig.

4s) and the detector is caused by movement ofv This is accomplished bymounting all high frequency circuits, namely, transmission plunger 28.

line 36, impedance matching network 46-41-58 and the detector, on aunitary structure which moves with plunger 28. If such an arrangement isnot provided it is necessary to use flexible high frequency transmissionlines and it has been found that movement of such lines causessufficient detuning to-give inaccurate or erratic results. y

In making impedance measurements by the resonant current method it isnecessary to be able .to measuresmall movements of plunger 28 with ahigh degree of accuracy.` This is accomplished by using a highlmultiplication gage 63 mounted on a metallic block 65 which rests on thesurface of base L In order to measure a small movementof plunger 2U' thecontact point of'gage 83 is placed against extension 24 in the directionof motion and thegage zeroedor its reading noted.

After the plunger has been moved the distance` Willtzbe.- indicatedfbythergage either directlyaor'.

as' thevdierencelmitwo .readi.1'1'gs. :The blockia must have su'icientweight.toaremainstationary on the surfaceaof.bassin..

VIt .-is.Y .not essential; thatfthe `transmission. line. .2 be of the.zconcentricitypeas `in the. .embodiment that has been described. Othertypes of shielded high frequency linesmay be used. For example, Figs. 7,8 and 9 show a two conductor shielded transmission line Ysuitable `foruse as an impedance measuring line).v Referring to Fig. 7, which is asection taken on a horizontal. plane 'through the center of the line,there are shown the end 9 of the line and the plunger end. The line iscomposed of two conductors 10- and 'l1y and a cylindrical shield 12. Theconductors 10 and 'H are supported at the end 9' by insulator 13 and theimpedance to be measured is connected between conductors "IUand 'HVatthis end. The plunger 20 is similar to plunger 20 of Fig.4 except thattwopassagewaysare provided for conductors 10 and H and the passagewayfor transmission line 38' is'located at 'the centerof the'plunger. Thetransmission line 36'fi`sa`lso of the two conductor type as-shown.The'end of plunger 20' is plated on the outside Vand' also on the insideof the passageways for conductors 10 `and H with a Vhard metal which ismachined and polished to give a very close and accurate ltbetween theinside of the shield and the end portion of the plunger and .between theoutside of conductors 10 and N'H and the inside of their Apassageways`at the end of the plunger. This construction is the .same as describedin connection withv Fig. .4 and provides la good short circuit betweenconductors l5 and 1| and between each of these conductors and theshield. As in the case of transmission 1ine'2 the shielda-nd twoconductors are made of highlyconductivematerial such ras brass which may-be plated with silver-if desired to further reduce losses.

-The coupling `probe carried by block t (Figs. 1 and 2) may,-in the caseofthe vtwo conductor lines, be iof` the form shown in Fig, 8. Or thecoupling may be accomplished by a small hori-` zontal loop asshowninFig. 9.

1Before describing-the manner'of using theI impedance Imeasuring linethe theory underlyingthe measurement vof limpedances bytheresonant-current and standing-wave methods will be briefly uring lineacross the end a, b of which the impedance to be measured 82 isconnected. Thev line is also provided withL an adjustable shortcircuiting means 83. The detector 84 and meter 55 are'loosely coupled totheshort circuited end of the line so that the reading of meter 85 isprom portio'nal to the .current in the line at this end.`

dicated Abyvmeter 85 will pass through a number cf maxima one of whichis as shown by the curve 88 in Fig. 11. If a 'constant excitation isapplied to the line by generator 86 and if the line losses are very lowthe curve 88 will' be symmetrical about the resonant .length of lineLras` shown. Also the distance between successive maxima will be'one-'half the lwavelength of the energy flowing in the transmissionline.

From measurements made on the system shown in Fig. the value of theimpedance 82.may be determined from the-.following equations: 5 l-k L(1) I Z-Zom u Z=impedance to be measured.

Zo=characteristic impedance of the impedance measuring line.

lc=complex reflection coeiilcient associated with the impedance Z.

kd=complexreflection coeflicient associated with the detector end of theline. Y

Q=an arbitrary real number greater than unity.

w=width of resonance-current curve 88 between points on opposite sidesat which 111%' (see Fig. 11) i=wavelength of exciting in thetransmission line. 0=phase angle oflc. 0d=phase angle of kd. Lr=lengthof line at which |I| is maximum (see Fig. 11)

In using the above equations it is first necessary to determine thevalue of lkdl and 0d in equations 2 and 3. The coecient lcd has'amagnitude different from unity since the element 83 does not produce aperfect short circuit due principally to the effect oi the detectorcoupling at this end of the line. The value of llccl may be determine byreplacing Z between points a, b in Fig. 10 with a short circuit andadjusting L to determine Le and w for a selected value of Q and todetermine A which is twice the'distancebetween successive maxima. Forthe short circuited condition k=l and |Ic|=1 since high frequency energyand Z, which is the impedance between points, a, b, is zero for a shortcircuit. Therefore` by substituting the measured values of w, a and|7cl=1 in Equation 2 the value of lkd| may be determined, and bysubstituting the measured value of Lr and A in Equation 3 6d may bedetermined since the phase angle 9 of Ic for a short circuit is 0. Theshort circuit may then be replaced by the impedance to be measured Z atpoints, a, b, and L again adjusted to determine the valuesof Lr and wforthis condition. Substituting this value of w together with the abovedetermined value of [kal in Equation 2 allows this equation to be solvedfor kl. Also substituting the value of Lr together with thevabovedetermined value of 6d in Equation 3 gives the phase angle 0. Thecomplex reflection coeicient associated with Z is now determined and isBy substituting the above value of 1c and the value of Z0 in Equation 1the value of Z may be determined. It is not necessary to recompute [kaland @d for each measurement since these factors do not change so long asthe electrical properties of the measuring line and its associateddetector, and the frequency at which measurements are being made remainunchanged.

In using the line shown in Fig. 1 to make measurements by theresonant-current method a signal generator is connected to terminal l5and a direct current or audio frequency indicator or meter is connectedto terminal B2. The signal generator and the probe l2 carried by block 6(Fig. 2) correspond to signal generator 8E and loop 8l of Fig. l0. Themeter connected to terminal 62, the detector contained in housing 55,the impedance matching device and transmission line 36 (Fig. 4), andloop 33 correspond to the meter 85, detector 84 and the coupling meansbetween the detector and shorting means 83 of Fig. 10. The position ofprobe I2 along the line should be such as to provide a suitable deectionof the meter connected to terminal 62 with minimum coupling between theprobe and the line. After the proper position of the probe has beendetermined the slot 5 on either side of block 6 is closed by slot coversIl (Fig. 3) to preserve the uniformity of the line.

The impedance to be measured is connected between the center and outerconductors at end 9 of the transmission line. The plunger is thenadjusted by hand wheels 25 and 29 until the meter connected to terminalB2 shows a maximum reading. The value of Lr may then be read from scale'15. To determine w the gage 63 is used in the manner already describedto accurately measure the distance between the two points at which theindicating meter reads values of current equal to the maximum readingobtained at Lr divided by Q (Fig. l1). The measured distance betweenthese two points is the value of w. To determine A the values of Lr fortwo successive maxima are noted on scale 'l5 and twice the distancebetween these two points is taken as i. These values of w, Lr and A maybe substituted in Equations 2 and 3 to determine 1c which may be used inEquation l to evaluate Z as described in connection with Fig. 10. If[kaf and 0a in Equations 2 and 3 are not known they may be determined byshort-circuiting end 9 of the line and determining Lr and wl as abovefor this condition. Substitution of these values of w and Lr inEquations 2 and 3, in which lk|=1 and 6:0

for the short-circuited condition, gives the values of licei and 0d, asexplained in connection with Fig. 10. As previously stated it is notnecessary to recompute lkdf and 0d unless the electrical characteristicsof the transmission line and the associated detector circuit or thefrequency is changed. y

There are some instances in which it is advantageous to makemeasurements by the standing-wave method, and, as has been stated, theline of Fig. 1, although primarily intended for use in accordance withthe resonant-current method, may also be used in accordance 'with thestanding-wave method. When used in this way the equivalent circuit ofthe line is as shown in Fig. 12. In this gure and 8i' represent the twoconductors of a transmission line which is excited at one end by agenerator having a generated voltage Eg and an internal impedance Zg.The impedance Z to be measured is connected across the other end of theline between points asraree a" and b'. internal impedance that itspresence across the line has no appreciable effect on the line, isarranged to be moved along the line to measure the voltage at Variousdistances from the load end. As the voltmeter is moved along the linethe voltage varies between maxima and minima as shown in Fig. 13 and ifthe attenuation in the line is negligible, lthe distance betweensuccessive maxima or minima is one-half the wavelength of the energy inthe line. i

The impedance Z under measurement may be obtained from the followingrelations:

Z--impedance to be measured Zo=characteristic impedance of measuringline k=complex reflection coeiicient associated with Z lk1=magnitude ofk =phase angle of 7c Emw=maximum voltage on line Emin=rninimum voltageon line Umin=distance from load end of line to a voltage minimum Emx,Emis, Umm and x may be determined by measurements made on the line usingtravelling voltmeter 9 i.

In using the line shown in Fig. 1 in accordance with the standing-wavemethod a signal generator is connected in place of the detector inhousing 55 so that energy is fed through the impedance matching networkM-iiS-d to transmission line 3S in housing fill, and thence throughtransmission line 35 to loop 33 in the face of plunger 20 (Figs. 4 and5) which serves to energize transmission line 2. That part oftransmission line 2 within block 4 corresponds to the transmission lineof Fig. l2 so that, in this case, that part of transmission line 2external to block ll merely serves to convey energy to that part insideblock 4. Therefore looking to the right into transmission line 2 at itspoint of entry into block 4 there appears a voltage Eg and an impedanceZg as in Fig. 12. The impedance to be measured is connected across thetransmission line at end 9.

The block 6 carrying the probe is replaced by the arrangement shown inFig. 14 for standingwave measurements. Referring to this gure, block 6carries a probe i2 which extends through slot into transmission line 2.The block 6 moves along block l as does block 6 in Fig. 1 and alsocarries a pointer so that the distance of the probe i2 from the end 9 ofthe line may be determined from scale ii (Fig. 1). The block 6 alsocarries a detector for producing a direct current voltage from the highfrequency energy picked up by probe l2'. The detector may be of anysuitable high frequency type and its details are not a part of theinvention. One suitable type of detector is shown in Fig. 14 andconsists of a crystal lill) electrically connected to the probe I2 andmounted in a 'threaded base li by means of insulator iii?! which alsosupports probe I2. -A bracket H33 also electrically connected to the Avoltmeter 91, having such high 1.0 crystal and s1`1ppcrtedV by insulator|112 serves to support 'thecentral-lconductor F04 of a concentrictuninglinethefouter conductor of which is formed by the tube |05. -Acat-Whisker |116 is electrically connected to base I-Ill andcontacts'the sur-face yof crystal |500'. The tuning line isadjusted'to'neara quarter wavelength at the-operating frequency ibymeans of 'adjustable lsnorting means IUS and together with the capacityFill between the outer conductor and the base IUI vforms a low pass lterbetween the crystal and meter |02 which allows the -direct current oraudio frequencyoutput of the detector but not the high frequency currentto reach D. C. or audio frequency meter 10B. Threading of the base IUIinto block 6' permits adjustment of the amount of probe 12" extendinginto transmission line 2. It will rbe vnoted that block 6" does not havea tongue `to v'ii-t slot5 as does block '6 (Fig. 2). Also slot coverssuch yas shown at II'I in Fig. 3 are not used in this application of'the line.

With the detector and indicator arrangement replacing block 6 in Fig. 1the impedance to be measured vis connected across end 9 of line 2. WithLa`-`constant Iexcitation applied to the line, block 6 (Fig. 14) isadjusted along that part of the line within block 4 to determine Emmi,Emis, Umm and A, the latter two measurements being made on scale 8.After these values have been determined the unknown impedance may beevaluated using Equations 4 through l as already explained.

I claim as my invention:

1. In an impedance measuring device a high frequency shieldedtransmission line, said line having an exposed end to which an impedanceto be measured may be connected, a slot in said shield extending fromsaid exposed end to an intermediate point on said line, means forpositioning a rst electrical coupling device through said slot into theinterior of said shield and for moving said coupling device to anyposition along said slot, means for short-circuiting the conductors ofsaid line, said short circuiting means being adjustable over theunslotted portion of said line whereby the length of said line betweensaid exposed end and said short-circuiting means may be varied,lmeansfor accurately measuring the length of said line between said exposedend and said short-circuiting means and means for accurately measuringsmall changes in said length, a second electrical coupling devicemounted on said short-circuiting means and movable therewith, and meansfor connecting each of said coupling devices to an external circuit.

2. Animpedance measuring device comprising a concentric transmissionline having a length several times. the wavelength of the frequency atwhich measurements are to be made, said line having an exposed end towhich an impedance to be measured may be connected, a slot in the outerconductor of said transmission line extending from said exposed end toan intermediate point on said line, means for positioning a firstelectrical coupling device through said slot into the interior oi saidline and for moving said first coupling device to any position alongsaid slot, means adjustable over the unslotted portion of said line forshort-circuiting the inner conductor to the outer conductor of saidconcentric transmission line, said short-circuiting means comprising acylindrical plunger made of highly conductive material and having acentral cylindrical passageway for the center conductor of saidconcentric transmission line, the inner end of said plunger having aplane face perpendicular to the longitudinal axis of the plunger, meansmaking good electrical contact between the plunger and the inner surfaceof the outer conductor and tlie` outer surface of the inner conductor ofsaid con-4 centric transmission line, an electricalcoupling loop mountedon saidy plane face of the inner end of said plunger, an additionallongitudinal passageway in said plunger, a transmission line connectedto said loop and extending through said additional passageway past theouter end of said plunger, a carriage movable relative to saidconcentric transmission line, meansv for supporting the outer end ofsaid plunger on said carriage, a rectifying device and an impedanceline, means for accurately measuring the length of said transmissionline between said exposed end and said short-circuiting means and meansfor accurately measuring small changesin said length.

-MING S. WONG.

REFERENCES CITED The following references are of record in the le ofthis patent: f

` v .UNITED STATES PATENTS Number Name Date 2,358,462 Mahren Sept. 19,1944 2,379,047 Thomas June 26, 1945 2,404,797 Hansen July 30, 19462,428,287 Linder Sept. 30, 1947 2,438,932 Mahren Apr. 6, 1948 OTHER'REFERENCES Meagher et al.: Practica] Analysis of Ultra High Frequency,R. C. A. Service Co., Inc., 2nd edition, August 1943, page 20.

Proceedings of the I. R. E., vol. 33, No. 9, September 1945; pages609-619.

